Data from several cancer screening studies are being collected and analyzed to gain a better understanding of the impact and consequences of such screening in various population settings, and to develop new techniques for data analysis. Section staff are involved in design, monitoring, and data analysis aspects of these studies. Screening Section investigators collaborated with the Early Detection Branch and the Research Contracts Branch in developing the major components of the PLCO Trial. This is a major trial of cancer screening in males and females for four cancers that comprise more than 50% of the incidence and mortality of cancer: lung, prostate, colorectal, and ovarian cancers. The trial design calls for a total sample size of 74,000 males and 74,000 females between the ages of 60 and 74 who are to be divided at random into a screened group and a control group. The screening techniques to be used are annual digital rectal examination and prostate specific antigen for prostate cancer, annual chest film for lung cancer, three-yearly flexible sigmoidoscopy for colorectal cancer, and annual ovarian physical examination plus CA-125 marker and transvaginal ultrasound for ovarian cancer. The Pilot Phase was completed in September 1994, during which 12,086 participants were enrolled and 4,818 screened. The Main Phase commenced in October 1994 with increased enrollment and screening. Data forms were modified and computers, hardware and software upgraded. Eligibility criteria were revised, quality assurance practices reassessed, and emphasis placed on minority recruitment. Preliminary data on recruitment, compliance, contamination, population characteristics, test results, and cancer cases were analyzed. The database from the HIP breast cancer screening trial was used to address several scientific and modeling issues. Issues under investigation included the magnitude and duration of benefit, age-specific effectiveness, and application to model development and validation. The colorectal cancer screening trial at the University of Minnesota is designed to evaluate testing for occult blood in the stool as an early detection maneuver for colorectal cancer. Staff completed their involvement in monitoring this trial which reported a 32% reduction in colorectal cancer mortality in the screened arm and assisted with long- term followup designs. Staff are involved in an international collaboration for the evaluation of prostate cancer screening trials. Trial investigators from Europe and North America are involved in an effort to develop a common case protocol and database so that in future years, data from various trials can be effectively combined to provide a more comprehensive evaluation of prostate cancer screening than could be achieved by any single trial.